Gas-filled discharge overvoltage protector

ABSTRACT

A spark gap device includes an open-ended cylindrical envelope of insulating material which is vacuum-sealed at its ends by electrodes. The electrodes are made of copper with the discharge surface of at least one having a layer of tin.

United States Patent 11 1 Schleimann-Jensen GAS-FILLED DISCHARGE OVERVOLTAGE PROTECTOR [75] Inventor: Carl Ame Schleimann-Jensen,

Danderyd, Sweden [73] Assignee: Telefonaktiebolaget LM Ericsson,

Stockholm, Sweden [22] Filed: Nov. 23, 1973 [21] Appl. No.: 418,611

[52] US. Cl. 313/325; 313/218; 313/311; 313/355; 317/62 [51] Int. Cl. ..H01J 17/00; 1101.1 21/00 [58] Field of Search 313/325, 306, 311, 179, 313/218, 355; 317/61, 62

[56] References Cited UNITED STATES PATENTS 2,365,518 12/1944 Berkey et a1 313/325 X 1 51 Sept. 9, 1975 2,457,781 12/1948 Mcttcn et al. 313/355 X 2,698,402 12/1954 Graham ct a1... 313/306 X 3,229,146 1/1966 Linkroum 313/306 X 3,374,382 3/1968 Young 1 313/179 X 3,588,576 6/1971 Kawiecki 313/325 3,676,743 7/1972 Peche et a1. 313/218 3,691,428 9/1972 Pcchc ct a1. 313/218 Primary Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or Firm-Hane, Baxley & Spiecens [5 7] ABSTRACT A spark gap device includes an open-ended cylindrical envelope of insulating material which is vacuumsealed at its ends by electrodes. The electrodes are made of copper with the discharge surface of at least one having a layer of tin.

4 Claims, 3 Drawing Figures GAS-FILLED DISCHARGE OVERVOLTAGE PROTECTOR One of the most usual forms for gas-filled discharge devices for preventing transient phenomena constitutes of a pair of electrodes disposed at a suitable distance from each other within a casing under high vacuum to form a discharge region, confining a gas of suitable kind and with a suitable pressure. lnsead of twoelectrode devices, three-electrode devices are sometimes preferred. Such three-electrode devices have a center electrode arranged between two end electrodes. The centre electrode can be pierced so that a common discharge region is provided for all the three electrodes. Valves or devices for protection against transients can be applied between terminals, which may be subject to overvoltages, or between such terminals and ground. When three-electrode valves are used, the centre electrode is usually connected to ground.

The firing voltage of the valve is determined from, among other things, the electrode material, the distance between the electrodes and the kind and pressure of the gas. The firing voltage is chosen so that the valve does not fire during normally applied voltages. On the other hand, the valve must fire if an overvoltage arises, otherwise the equipment can be damaged. The discharge takes place first as glow discharge, and the voltage across the valve and therewith across the protected equipment is then limited to the glow voltage of the discharge valve. The discharge passes, however, to are discharge, if a greater amount of energy is available during the overvoltage. The voltage across the valve and therewith across the equipment then drops to the are voltage level, which is considerably lower than the glow voltage. A valve with a nominal firing voltage of 350 volt can thus have a glow voltage of about 175 volt and an arc voltage of -50 volt. If the overvoltage appears in form of a transient with limited contents of energy, it is possible that the energy is dissipated in connection with the firing of the valve and the following glow discharge. If the transient has a greater energy content, a glow discharge appears first; but this passes to a discharge current of about 0.5 ampere in are discharge, with the energy in the transient being rapidly dissipated. The valve is extinguished when the energy of the transient have been dissipated and normal voltages again are present, otherwise the valve has only glow discharge or it continues with are discharge. The transients to be protected against have often very fast rise time. Rising slopes of about 500-5000 volt per micro second are not unusual. A good protective transient valve must therefore be fast operating so that it functions before the voltage has had time to rise to such values that the equipment is damaged. It is known that discharge valves with copper electrodes arranged so that the periphery of the discharge gap is close to a ccramic cylinder which surrounds the electrodes, work extremely rapidly, if a smaller amount of copper is dusted on the ceramics from the electrodes. It is considered that valves of this kind during rapid overvoltage transients fires as a result of field emission within the area outside the electrode gap where copper has been dusted on ceramics. The electrons, which are detached during field emission rapidly rise in voltage and this results in a very rapid ionization of the gas in the discharge gap, which again results in a very rapid firing of the valve. The smaller the distance from the copper electrodes to the ceramics is the faster the valve operates. The good hcat conductivity of the copper causes the heat from the discharge area itself to be rapidly distributed to the whole electrode. This results in a great loading capacity in transient connections. In many cases it is enough to protect the equipment against damage from ovcrvoltage transients. Discharge valves with copper electrodes surrounded by a ceramic body have in such cases an excellent good protection effect because of their fast operation and loading capacity.

However, there are cases when equipment must also be protected against discharges of longer duration. If, for example, aerial lines are used, there can arise both overvoltage transients (among other atmospheric discharges) as well as more lasting alternating discharges (for example short-circuits to power lines or induction from these). In such cases the equipment must be pro tected during the time required for fuses or ovcrcooling current relays to operate. This means that the valve for about half a second can be loaded with an alternating current of several ampere. Since the arc voltage of valves with copper electrodes and normally used gap dimensions is about 45V, the heating in the discharge valve can be rather large. Sometimes the electrodes of the valve must be provided with cooling means to dissipate the generated heat othcrwise because of a certain difference in the thermal expansion between the copper electrodes and the ceramic casing joined with the electrodes there can arise ceramics cracks which destroy the effectiveness of the valve.

A lowering of the are voltage results in a lower thermal effect for a certain current. Therefore there has become available many types of valves with electrodes, which are activated by covering with material with low electronic ionization energy, for example barium oxide or thorium oxide. Sometimes powder of nickel or another metal is mixed in to increase the conductivity of the covering. In such connections KOVAR has often been used as electrode base material. In activated valves of this kind, are voltages of about 15 volt can be achieved. This fact in combination with the relatively small ability of the KOVAR-material to draw heat from the discharging gap to the area of the electrode-isolator junction, and the heat expansion of KOVAR-material which is similar with glass and ceramics, results in advantageous thermal capacity for alternating current compared with valves of the same dimensions provided with copper electrodes. At the same time there arises however a risk of extreme heating of the gap area of the electrodes because of the small capacity of heat con ductivity of the KOVAR-material. This can result in such a great volatilization of the material that the elec trodes are punctured, or that material condenses on the isolating part of the valve causing a fault in the insulation. In both cases the valve becomes useless for its intended purpose.

Valves with copper electrodes are, however, rather superior to the previous mentioned activated valves with respect to the speed of operation for transient protection. Among other things radioactive materials have been added, or radioactive gas has been used in valves with activated electrodes. A certain improvement of the operating speed can be achieved, but not to the same low level which can be achieved with copper electrodes. In many connections, however, there are risks or inconveniences in using components with radioactive material or radioactive gas. In valves according to the present invention it has been possible to combine the fast operation of ceramic-discharge valves provided with copper electrodes in response to transient with more loading capacity for alternating current without the use of either radioactive material or gases or electrode covering with material, which have a 'low electronic escape energy. The valves according to the invention have the further advantage compared to valves with KOVAR-electrodes that the loading capacity can be increased further with the help of cooling means. The rather small heat conductivity of the KOVAR- electrodes results in a negligible increase of the loading capacity by using cooling means.

The characteristics for a discharge valve according to the invention appear from the appended claims. In valves according to the invention copper electrodes are, for example, used, which at least in the gap surfaces are coated with a layer of suitable metal with low arc voltage and with a thickness up to a few tenths of a millimetre. To maintain the speed of the valve during firing it is desireable that the coating metal during heating forms an alloy with the electrode material. With consideration to the isolation requirements imposed on the valves, it is furthermore desireable that the used coating metal or its copper alloy during temperatures up to about 650C (degassing temperature during production) has a boiling pressure below about millimetres Hg, and that the boiling point (eventually the sublimation temperature) for the coating metal or the alloy is higher than the maximum temperature used in the production of the valves, i.e., about 825C. Bismuth, antimony and tin are proved to be suitable met als in this connection, but also other metals may be considered. However, tin is preferred. It has proved that copper electrodes with such metal coating operate fast enough for transient overvoltages and also have great capacity for alternating current. The heat, which arises during an arc discharge is distributed rapidly over the whole electrode because of the high heat conductivity of the copper. Such heat eventually flows to a cooling means combined with the electrode. In connection with are discharges, continuously migrating cathode and anode spots of a diameter of one or a few tenths of a millimetre arise where the temperature momentary can reach up to several thousands of degree Celsius. The metal vaporization, which this heating results in, is necessary for the existing of the are discharge. The heating, however, also causes an alloy to be formed between the copper material of the elec trode and the metal brought on the gap surface. This alloy formation previously began when the valves were exposed to the temperature required for the joining together of the electrodes and the ceramic body. The bronze, which consequently is formed at the alloy process, seems to have practically the same low arc voltage as the applied metal, but the fusing point is higher, which is advantageous.

Compared with earlier known valves with copper electrodes, the valves according to the invention with stand a threefold loading for alternating current. If the electrodes are provided with a suitable cooling means, the strength increases essentially more for alternating current loading.

The invention is described more in detail in connection with the accompanying drawing wherein:

FIG. 1 shows a two electrode valve according to the invention;

FIG. 2 shows an alternate electrode contouring according to the invention; and

FIG. 3 illustrates the application of the invention to a three-electrode valve.

In FIG. 1, there is shown in cross-section a discharge valve according to the invention having two electrodes E and E mounted in a vacuum tight enclosure by cylinerical ceramic body K. L and L are soldered seams between the units E,K and KE The soldering is carried out with help of a suitable metal solder. A copper-silver-solder is often chosen with a fusing point of about 800C. The soldering is suitably carried out in the gas which the valve will contain, and the gas pressure is adapted so that desired gas pressure is present in the valve after cooling the soldered valve. The magnitude of this gas pressure is about 50-100 millimetre Hg.

The electrodes E and E are of copper on which a coating S and S respectively of one of the metals bismuth, antimony, zinc or tin has been formed. The coating can be formed merely on a part of the electrode surface in the gap area G, as indicated by coating 8,, or it can in accordance with coating S cover the whole gap area of the electrode. Tests have also been carried out with electrodes where all existing electrode surface in the discharge region has been covered by a coating of the above mentioned low arc voltage metals as well as for the annular region between the electrode and ceramic body. With respect to the arc voltage this is quite acceptable, but the firingrcsponse during transients is slower when such totally covered electrodes are used.

It has been established that the thickness of the coating can preferably be of one or a few tenths of a millimetre. The method of application is of secondary importance. It can be done galvanically or by melting of suitable amounts of metal, for example in the form of suitable bits or amounts of powder. Also, alloys of the mentioned low are voltage metals, or of these and copper, can be used for the purpose. Powder can also be suspended in a liquid, to which is eventually added with binder, so that application to the electrodes is facilitated. As an example of a method for the production of discharge valves according to the invention the following is mentioned. Electrodes of the same form as shown in FIG. 1 are placed on a plate of heat-resisting material. The gap surfaces are turned upwards, and in every gap hollow there is placed a tin pellet of suitable size. Plates with the electrodes are placed in a furnace with suitable protection atmosphere, for example argon and hydrogen gas. During heating to about 230C the tin melts and flows out over an area, which is primarily determined by the amount of tin used. The temperature can then be increased to 600C for example, or even up to 800C, which corresponds to the soldering temperature used during the joining together of the valves. The result of the heat treatment is that predetermined parts of the surface of copper electrodes will be covered with a bronze coating. The electrodes are then used in the assembly of the valves.

Forming of a bronze coating is not a necessary condition to achieve discharge valves with good quality of protection. It is enough to let the low are voltage metal melt on the surface of the electrode. A certain degree of alloy with the copper of the electrode will, however, also in this case take place in connection with the soldering of the valve. Besides, it has been established that the supplying of desired metal simply can take place in connection with the joining together of the valves. An electrode for example electrode E in FIG. 1 can for example be placed on a plate. In the gap region of the electrode a pellet of desired metal is placed. The joining together continues with placing of a solder ring L ceramic body K, another solder ring L and finally an upper electrode E,, which has not been provided with coating material. In connection with the following soldering, the supplied metal melts in the lower electrode. A coating is formed and certain alloy with copper takes place in this electrode. in connection herewith a certain volatilizing of the supplied metal can take place to the upper electrode. More material will, however, be transported during discharges, which the valve will be exposed to during the phases of production, which follows after the soldering of the valve. Final inspections have shown that this simplified technic of production gives practically the same final result, which is achieved when pretreated electrodes are used.

It has before been mentioned that the load carrying ability during alternating current loading for valves according to the invention can be increased further with the help of cooling means. In FIG. 1 the ring formed surfaces of the electrodes are indicated with H and H which suitably can be combined with such means. These can suitably consist of copper cylinders.

In FIG. 2 is shown an alternative electrode form with a plane gap surface. Also here the whole surface, see S on the right side of the figure, or only a part of this, see on the left side, can be provided with a coating according to the invention.

FIG. 3 finally shows how a three electrode valve comprising end electrodes E and E centre electrode E and ceramic rings K and K according to the invention has been provided with low arc voltage metal S and S on a part of the surfaces of the electrodes. One or a few of the electrodes can be pretreated before the joining together takes place, but also in this case it'is possible to achieve good result by supplying the desired metal to only one of the electrodes (the lower) in connection with the joining together of the valve.

The details shown in the figures are drawn in enlarged scale. The proper dimensions for two electrode valves are 69 millimetre in diameter and 56 millimetre in height. Three electrode valves have mostly about the same diameter and double the height.

As examples of metals, which can be coated over respectively alloyed with for example copper electrodes bismuth, antimony, zinc, and tin have been mentioned. For these metals the middle are voltage coatings vacuum is between 5 over-voltages to 15 volt while the product of thermal conductivity and boiling point is between 10 and 1000 cal/ems.

I claim:

L'An overvoltage protector comprising at least first and second electrodes having faces disposed opposite each other across a gap, each of said electrodes being of a conductive material, the surface of at least a portion of the face of one of said electrodes being covered with a metal having properties such that the product of its thermal conductivity and boiling point is less than lOOO cal./cms. with an arcing voltage in vacuum of 5 to 15 volts, and a tubular envelope, each of said electrodes being vacuum-seal fixed into one end of said tubular envelope, respectively.

2. The overvoltage protector according to claim 1 wherein said metal is tin.

3. An overvoltage protector comprising at least first and second electrodes having faces disposed opposite each other across a gap, each of said electrodes being copper, the surface of at least a portion of the face of one of said electrodes being covered with a layer of tin.

4. The overvoltagc protector of claim 3 wherein said layer of tin is less than one millimeter.

I l l 

1. An overvoltage protector comprising at least first and second electrodes having faces disposed opposite each other across a gap, each of said electrodes being of a conductive material, the surface of at least a portion of the face of one of said electrodes being covered with a metal having properties such that the product of its thermal conductivity and boiling point is less than 1000 cal./cms. with an arcing voltage in vacuum of 5 to 15 volts, and a tubular envelope, each of said electrodes being vacuum-seal fixed into one end of said tubular envelope, respectively.
 2. The overvoltage protector according to claim 1 wherein said metal is tin.
 3. An overvoltage protector comprising at least first and second electrodes having faces disposed opposite each other across a gap, each of said electrodes being copper, the surface of at least a portion of the face of one of said electrodes being covered with a layer of tin.
 4. The overvoltage protector of claim 3 wherein said layer of tin is less than one millimeter. 